Band-gap modulation of two-dimensional saturable
absorbers for solid-state lasers
Shuxian Wang,
1
Haohai Yu,
1,2
and Huaijin Zhang
1,3
1
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China
2
e-mail: haohaiyu@sdu.edu.cn
3
e-mail:huaijinzhang@sdu.edu.cn
Received December 24, 2014; revised February 10, 2015; accepted February 12, 2015;
posted February 13, 2015 (Doc. ID 231453); published March 16, 2015
Due to the manifestation of fascinating physical phenomena and materials science, two-dimensional (2D) materi-
als have recently attracted enormous research interest with respect to the fields of electronics and optoelectronics.
There have been in-depth investigations of the nonlinear properties with respect to saturable absorption, and many
2D materials show potential application in optical switches for passive pulsed lasers. However, the Eigen band-gap
determines the responding wavelength band and constrains the applications. In this paper, based on band-gap
engineering, some different types of 2D broadband saturable absorbers are reviewed in detail, including molyb-
denum disulfide MoS
2
, vanadium dioxide VO
2
, graphene, and the Bi
2
Se
3
topological insulator. The results
suggest that the band-gap modification should play important roles in 2D broadband saturable materials and
can provide some inspiration for the exploration and design of 2D nanodevices. © 2015 Chinese Laser Press
OCIS codes: (140.0140) Lasers and laser optics; (140.3540) Lasers, Q-switched; (140.3380) Laser materials;
(140.3538) Lasers, pulsed.
http://dx.doi.org/10.1364/PRJ.3.000A10
1. INTRODUCTION
Lasers, since first demonstrated by a ruby crystal in 1960 [1],
have shown tremendous progress so far. On account of the
advantages of large pulse energy, short pulse duration, and
high peak power, pulse lasers, including mode-locking and
Q-switching, receive more widespread attention [
2,3]. Pulsed
lasers with the pulse width from nanosecond to subpicosec-
ond scales play a key role in the industrial machining, remote
sensing, and military defense areas [
4]. Ultrashort femtosec-
ond lasers have significant applications in the fields of the
in-situ detection, complex reaction dynamics, medical survey,
and telecommunication [
3]. Passive pulse modulation could
be operated with compact and cost-effective structures, such
that it has become an influential research direction in the laser
field. Taking advantage of saturable absorption of saturable
absorbers, that is part of the third-order nonlinear properties,
passive pulse modulation in the Q-switched or mode-locking
lasers could be easily implemented. Traditional saturable
absorbers, such as ion-doped crystals (Cr:YAG, Cr:ZnSe, V:
YAG, and so on) [
5–7], GaAs [8], and color-center crystals
(for example F
2
:LiF) [9], are wavelength-sensitive and are
only applied in their respective corresponding wavelength
bands. Although the commercial semiconductor saturable
absorber mirrors (SESAMs) could be fabricated for the
specific wavelength from 0.4 to 2.5 μm by using different
semiconductor material systems [
2,10], the relatively complex
structural design may increase the product cost and the diffi-
culty of preparation. In addition, the quasi-phase-matching
technique in optical superlattice, which is feasible for a wide
wavelength range as long as it is in the transparent spectral
region of the optical superlattice material, is also a promising
method to obtain passive mode-locking lasers [
11–13].
Nevertheless, this technique requires the high-efficiency
generation of second-harmonic generation and elaborate
design of the resonant cavity and the second-harmonic
crystal. Therefore, the development of low-cost and robust
broadband saturable absorbers is still an intense focus of
research nowadays [
2].
Graphene, as the representative of the two-dimensional
(2D) materials, since found by K. Geim and K. S. Novoselov,
has profoundly promoted and broadened the research areas
of 2D materials in the fields of electronics and optoelectronics
due to an abundance of fantastic physics [
14–23]. With in-
depth studies, many other novel 2D materials are discovered
and are brought into people’s horizons, such as transition
metal dichalcogenides (TMDCs) (typically MoS
2
, MoSe
2
,
WS
2
, and so on) [16,17,19,24,25], transition metal oxides
(for instance MoO
3
and TiO
2
)[17,24,26], topological insula-
tors (Bi
2
Se
3
,Bi
2
Te
3
, and Sb
2
Te
3
)[16,17,24,27], as well as sil-
icone [
28], germanane [16,28], black phosphorus [29,30], and
other graphene analogues (typically h-BN) [
19,24]. For the ma-
jor 2D materials, the layered structure results in the strong in-
plane coupling and the weak van der Waals coupling between
layers. Therefore monolayer or few-layer 2D perfect samples
could be easily fabricated by mechanical exfoliation or chemi-
cal exfoliation [
18,31]. In addition, aiming at the growth habits
and atomic arrangements for different 2D materials, many
more efforts are made to explore the economical and practical
growth methods, such as chemical vapor deposition, and epi-
taxial growth on adaptive substrates [
21,25]. The layered 2D
materials exhibit numerous exotic physical, chemical, and
mechanical properties such that they are rapidly developed.
Many potential functional applications in terms of 2D
materials in some aspects will be realized in the near future,
A10 Photon. Res. / Vol. 3, No. 2 / April 2015 Wang et al.
2327-9125/15/020A10-11 © 2015 Chinese Laser Press
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